PROJECT SUMMARY (Abstract) The classical Knudson?s two-hit model of tumor suppressor gene function envisions the sequential inactivation of both alleles as obligate steps in tumor formation and malignant progression. Epimutation (i.e. mitotically stable gene silencing associated with epigenetic alteration in DNA methylation) is proposed to act as one of the Knudson?s two hits for tumorigenesis. Remarkably, however, it remains unclear whether epimutation frequently observed in cancer cells plays a causal role in carcinogenesis. In human lung cancers, p16INK4A promoter hypermethylation is one of the earliest detected epimutations, and is thought to function as a critical initiating event in tumorigenesis. The importance of this epimutation is further underscored by the associations with distinct gene mutation patterns, cancer prognosis, and response to therapy. However, the molecular pathways linking aberrant DNA methylation, epigenetic silencing, and tumorigenesis remain poorly characterized. In this regard, we published the first mouse model demonstrating that targeted p16 epimutation drives spontaneous tumor development. The tumor spectrum of this epimutation includes sarcoma, lymphoma and lung adenocarcinoma. More importantly, our preliminary studies suggest that p16 epimutation operates synergistically with oncogenic K-RAS activation to drive lung tumor progression. Based on these findings, the overall goal of this proposal is to understand the role of p16 epimutation as a functional driver of lung preneoplasia. Specifically, we will: 1 ? Investigate the functional contribution of p16 epimutation in a mouse model of lung adenocarcinoma. 2 ? Investigate the therapeutic effects of reversal of p16 epimutation using CRISPR-based epigenetic editing. These studies will elucidate how DNA methylation, an important mechanism regulating the expression of tumor suppressor genes, drives lung tumorigenesis. The results derived from this proposal could have a unique impact in the design of cancer epigenetic therapies.